In the nuclear power industry it is extremely important to continuously monitor the status of the fuel assemblies used in a nuclear reactor. Examples of the parameters monitored are handling damage, oxide growth, presence of foreign objects and curvature.
In nuclear power reactors, the reactor core is comprised of a number of fuel assemblies. Depending on the size of the reactor, there can be for example 100 or more fuel assemblies in the core at any time. Since the fuel assemblies are densely packed in predetermined positions in the reactor core, dimensional standards of each fuel assembly must be met within very close tolerances. Pre-service quality assurance inspections are performed to ascertain any deviations in the formation and assembly of new fuel assemblies from accepted tolerances. The fuel assembly is also inspected to determine whether it is straight, unbowed, and without any twist.
During reactor operation, the fuel assemblies become irradiated/irradiating and can become bowed and twisted due to differential growth resulting from high temperatures, temperature gradients, pressure, vibrations and water flow. During each reactor refueling cycle, fuel assemblies are moved to different locations in the reactor core, with some of the fuel assemblies being replaced by new fuel assemblies. Certain fuel assemblies become spent and are removed from the reactor. Since at least a portion of the irradiated fuel assemblies are moved from one location in the reactor core and eventually to another location in the core during refueling operations, there is a need to verify the continued serviceability of these fuel assemblies.
Thus, the accurate and efficient determination of deviations from dimensional standards of the fuel assembly as well as whether the fuel assembly is unbowed and without twist is particularly important for irradiated/irradiating fuel assemblies which have to be inspected and remotely manipulated to protect personnel against exposure. Furthermore, there is an increased risk of damaging a fuel assembly when it is moved, for example lifting it from its position in a core into an inspection stand.
Deformation of the fuel assembly can prevent the accurate alignment of the fuel assembly with the lower core plate, cause interference with adjacent components/fuel assemblies, and in extreme cases make it impossible to insert the control rods, which is a reactor safety issue. In addition, the deformation of the fuel assembly is frequently not perceived until the fuel assembly is attempted to be placed in its constrained position within the reactor core.
In some prior art solutions, bow or curvature is measured by relocating a fuel assembly to an intermediate location to perform the measurements, leading to very expensive down-time if the fuel assembly is in use at a nuclear power plant. A fuel assembly that is located near a control rod and is too deformed, for example to bent, must be replaced, which is a time consuming and costly process. Due to the high cost for down-time, only a selection of the fuel assemblies can be measured using prior art solutions and the state of the remaining fuel assemblies are not considered or approximated from previous measurements of those fuel assemblies. In some embodiments, the majority of the fuel assemblies at control rod positions are measured, but not many of the remaining fuel assemblies. As selection is typically manually performed, there is always a risk of human errors, for example forgetting a fuel assembly that should have been measured.
Measurement of deformations are typically performed using a physical reference object, for example a wire, placed relative to the fuel assembly, and distance comparisons are then performed manually, through visual inspection of the fuel assembly relative to the reference object, or using a distance measurement device.
There is a need for a deformation measurement method that is less expensive, less time-consuming, reduces the risk of introducing human errors, poses a lower risk of introducing additional deformations during measurement, and/or enables accurate measurement of all, or at least most of, the fuel assemblies in a reactor core.